In general, a resolver is provided with an inner coil portion rotatably disposed inside an outer coil portion. The resolver detects a rotational angle of a rotor based upon a change of a mutual inductance between the inner coil portion and the outer coil portion. Therefore, as described in Japanese Patent Laid-Open Publication No. 1998-328952, this type of resolver is applied for a servo motor and acts to detect a rotational angle of a motor shaft.
Recent developments have lead to a torque sensor capable of detecting a torque corresponding to a torsion angle of a torsion bar rotatably disposed in a housing based upon an angle difference between a first angle detected by a first resolver equipped at one end of the torsion bar and a second angle detected by a second resolver equipped at the other end of the torsion bar.
More particularly, in the resolvers of the aforementioned torque sensor, a rotor having the inner coil portion and a stator having the outer coil portion are arranged to match a reference point of the rotor with a reference point of the stator. The first resolver is equipped at the one end side of the torsion bar, while the second resolver is equipped at the other end side thereof. In assembling this torque sensor, the stators of the first and second resolvers are positioned so as to match a reference position of the stator of the first resolver with a reference position of the stator of the second resolver. The first and second resolvers are then equipped at the housing. The rotors of the first and second resolvers are also positioned so as to match a reference position of the rotor of the first resolver with a reference position of the rotor of the second resolver. The first and second resolvers are then equipped at the torsion bar. Following to the above processes, the torsion bar is rotatably disposed in the housing so as to build the torque sensor. In this case, the stators are equipped at the housing by a positioning jig. Therefore, the stators having the outer coil portions can be relatively highly precisely assembled with the reference positions matched. However, the rotors having the inner coil portions may not be positioned at the right reference positions due to torsion of the torsion bar upon assembling the rotors at the torsion bar. Further, each rotor may contain variations upon being manufactured, wherein a deviance between the two rotors may be further increased. As a result, an unnecessary angle difference may occur between the angle detected by the first resolver and the angle detected by the second resolver, wherein a torque of the torsion bar may be calculated counting in the unnecessary angle difference.
For example, when an electric power steering device for a vehicle is provided with this type of torque sensor having the relatively large angle difference, a torque sensor adjustment or a vehicle adjustment by a vehicle manufacturer may not be able to be effectively performed due to a reaction torque corresponding to the torque calculated counting in the unnecessary angle difference. Conventionally, this sort of problem has been prevented from occurring by defining a certain standard that can be a parameter for judging whether the torque sensor is a finished product or a not applicable product. When the torque sensor does not satisfy the standard, the torque sensor has been discarded.
However, it has been difficult to assemble the rotor with high assembling precision in accordance with the conventional assembling method. Therefore, even if the certain standard is defined as described above, it has been still difficult to satisfy the standard, thereby deteriorating a production capability, discarding the torque sensor not satisfying the standard and causing excessive manufacturing cost.
A need exists for providing a method of assembling a torque sensor, a torque sensor and an electric power steering device, each of which enables reducing a manufacturing cost and improving a production capacity.